How can I install a walk in tub shower myself?

I assume you mean in a finished house.This is gonna get messy.Most bathrooms have a 24 in.door, which means a 32 in.unit will not go thru.you will have to gut the door.Then there's the length to contend with.Tubs are set in place before sheetrock is set,so you'll have to gut the rock as well.The other option is to go thru the outside wall.the third option is to get a unit that comes with the walls separate from the tub,with less demolition.When the tub is in, place a small torpedo level on the floor ,side to side. When level, nail or screw in place.